The neutron detector is an elemental technology which supports the technology using neutrons, and along with the development of the technology using neutrons in the security field such as cargo inspection or so, the academic research field such as the structural analysis by a neutron diffraction or so, a nondestructive inspection, or the medical field such as boron neutron capture therapy or so, the neutron detector having higher performance is in demand.
As the important characteristic which is required for the neutron detector, the neutron detection efficiency and the discrimination ability between neutrons and γ ray (hereinafter, it may be referred as n/γ discrimination ability) are mentioned. The neutron detection ability refers to the ratio of the numbers of neutrons counted by the detector with respect to the number of neutron entered in the detector; and if the neutron detection efficiency is low, the absolute numbers of neutrons being counted will decline, hence the counting accuracy will decline. Also, γ ray exist as a natural radiation, and further it is generated when neutrons contact with the constituting member of the detector system for detecting neutrons or when neutrons contact the inspection objects; thus if the n/γ discrimination ability is low and γ ray is counted as neutrons, then the neutron counting accuracy will decline.
In case of detecting neutrons, since neutron has strong force to transmit through the material without causing any mutual interaction in the substance, it is generally detected using the neutron capture reaction. For example, helium 3 detector is known conventionally which detects using proton and tritium generated by a neutron capture reaction between helium 3 and neutrons. This neutron detector is a proportional counter tube filled with helium 3 gas which has high detection efficiency and excellent n/γ discrimination ability; however helium 3 is expensive substance and furthermore the resources are limited.
Recently, instead of the above mentioned helium 3 detector, the neutron detector using the neutron scintillator has been developed. The neutron scintillator refers to the substance which emits fluorescent light when neutrons enter due to the effect of said neutrons, and by combining the photodetector such as a photomultiplier tube or so with the neutron scintillator, the neutron detector can be made. Note that, the aforementioned various performances of the neutron detector using said neutron scintillator depends on the substance constituting the neutron scintillator. For example, if large amount of the isotope having high neutron capture reaction efficiency is comprised, then the detection efficiency against neutrons will be enhanced. As such isotope, lithium 6 or boron 10 or so may be mentioned (for example, the patent document 1).
In said neutron detector, the photodetector will detect the light emitted from the neutron scintillator, and the pulse form signal will be output from the photodetector. In general, the numbers of neutrons are measured by the intensity of the pulse form signal so called a pulse-height. That is, for said pulse-height, a predetermined threshold is set, and the event showing the pulse-height higher than said threshold is counted as the neutron incident event; on the other hand the event showing the pulse-height lower than the threshold is considered as a noise. Therefore, in order to enhance n/γ discrimination ability, it is important to reduce the pulse-height generated by γ ray incident.
Also, in said neutron detector, in order to collect the light from the phosphor having large sensitive area, or to provide the position resolution to the neutron detector, the wavelength converting fiber is used (see the patent documents 2 to 5).